U.S. patent number 6,861,103 [Application Number 10/360,443] was granted by the patent office on 2005-03-01 for synthesis of functional polymers and block copolymers on silicon oxide surfaces by nitroxide-mediated living free radical polymerization in vapor phase.
This patent grant is currently assigned to The Regents of the University of California. Invention is credited to Ying Chih Chang, Xiaoru Chen, Jun Li.
United States Patent |
6,861,103 |
Chang , et al. |
March 1, 2005 |
Synthesis of functional polymers and block copolymers on silicon
oxide surfaces by nitroxide-mediated living free radical
polymerization in vapor phase
Abstract
Nitroxide mediated free radical polymerization of vaporized
vinyl monomers, including acrylic acid (AAc), styrene (St),
N-2-(hydroxypropyl)methacrylamide (HPMA) and N-isopropyl acrylamide
(NIPAAm), on silicon wafers is demonstrated. FTIR, ellipsometry and
contact angle goniometry were used to characterize the chemical
structures, thickness and hydrophilicity of the films. The growth
of film is linearly proportional to its reaction time, leading to
the easy and exact control of polymer film thickness from
nanometers to submicrons. The capability of polymerizing various
monomers allows us to fabricate various functional polymer brushes.
The reversible thermo-responsiveness of a 200 nm thick grafted
poly(NIPAAm) film in aqueous solution is demonstrated with over 50%
change in thickness at its lower critical solution temperature. A
tri-block copolymer of poly(AAc)-b-polySt-b-poly(HPMA) is
successfully synthesized, proving the renewability of
TEMPO-mediated polymerization at vapor phase. Surface polymer
composition and morphology is thus controlled at nanoscale by
utilizing vapor phase surface-initiated controlled
polymerization.
Inventors: |
Chang; Ying Chih (Atherton,
CA), Li; Jun (Irvine, CA), Chen; Xiaoru (San Diego,
CA) |
Assignee: |
The Regents of the University of
California (Oakland, CA)
|
Family
ID: |
29553173 |
Appl.
No.: |
10/360,443 |
Filed: |
February 7, 2003 |
Current U.S.
Class: |
427/522;
427/255.11; 526/304; 526/317.1; 526/303.1; 427/255.14; 427/487;
427/593; 526/220; 257/E21.259; 526/346 |
Current CPC
Class: |
B05D
1/60 (20130101); C08F 4/00 (20130101); C08F
283/12 (20130101); C08F 289/00 (20130101); C08F
293/005 (20130101); C08L 51/08 (20130101); C08L
51/085 (20130101); H01L 21/02118 (20130101); H01L
21/312 (20130101); H01L 21/02271 (20130101); C08F
2438/02 (20130101) |
Current International
Class: |
C08F
4/00 (20060101); B05D 7/24 (20060101); C08F
293/00 (20060101); C08F 283/12 (20060101); C08F
289/00 (20060101); C08L 51/08 (20060101); C08L
51/00 (20060101); C08F 283/00 (20060101); H01L
21/312 (20060101); H01L 21/02 (20060101); C08F
002/46 () |
Field of
Search: |
;526/220,303.1,304,317.1,346 ;427/255.11,255.14,487,522,593 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Pezzuto; Helen L.
Attorney, Agent or Firm: Dawes; Daniel L. Myers Dawes Andras
& Sherman LLP
Parent Case Text
RELATED APPLICATIONS
The application is related to U.S. Provisional Patent Application,
Ser. No. 60/355,733, filed on Feb. 7, 2002, incorporated herein by
reference and to which priority is claimed pursuant to 35 USC 119.
Claims
We claim:
1. A method for forming organic thin films comprising: providing a
substrate having a surface; covalently pre-immobilizing a
derivative of 2,2,6,6-tetramethyl piperidinyloxy (TEMPO) based
alkoxylamine containing trimethoxysilyl on the surface of the
substrate with the TEMPO group at the free end; and growing a
grafted polymer layer in vapor phase on the pre-immobilized surface
by means of living free radical polymerization.
2. The method of claim 1 where growing the grafted polymer layer in
vapor phase comprises growing a Poly N-isopropyl acrylamide
(PNIPAAm) film on the substrate in vaporphase.
3. The method of claim 1 where growing the grafted polymer layer in
vapor phase comprises growing a diblock polymer.
4. The method of claim 1 where growing the grafted polymer layer in
vapor phase comprises growing a triblock polymer.
5. The method of claim 1 where growing the grafted polymer layer in
vapor phase comprises linearly controlling the thickness of the
grafted polymer layer by controlling reaction time.
6. The method of claim 5 where linearly controlling the thickness
of the grafted polymer layer controls the thickness in the range
from 1 nm to submicron sizes.
7. The method of claim 1 where growing the grafted polymer layer in
vapor phase comprises utilizing nitroxide mediated, free radical
polymerization of vaporized vinyl monomers in vacuum to synthesize
the polymer thin film.
8. The method of claim 1 further comprising using photolithography
to create surface patterns in the thin film.
9. The method of claim 1 further comprising regulating surface
properties of the substrate through an environment stimulant.
10.The method of claim 1 where growing the grafted polymer layer in
vapor phase comprises forming a thin film of polymeric materials
including polystyrene (PS), polymers with functional groups such as
hydroxy, carboxy, and amide, and/or block copolymers composed of
more than two of the polymers with functional groups.
11. A method for forming organic thin films using surface initiated
vapor deposition-polymerization of monomers comprising: providing a
substrate having a surface; providing an initiator; predepositing
the initiator on the surface of the substrate; disposing the
modified substrate in a vacuum containing the monomer; activating
the initiator; vaporizing the monomer; and reacting the monomer to
the modified surface with the activated initiator present, where
providing the initiator comprises synthesizing
1-(4-oxa-2'-phenyl-12'-trimethoxysilyl
dodecyloxy)-2,2,6,6-tetramethyl-piperidine (I) (TEMPO).
12. The method of claim 11 where vaporizing the monomer vaporizes
acrylic acid (AAc) monomers.
13. The method of claim 11 where vaporizing the monomer vaporizes a
monomer selected from the group consisting of styrene (St),
N-(2-hydroxypropyl)methacrylamide (HPMA) and, N-isopropyl
acrylamide (NIPAAm).
14. The method of claim 11 where reacting the monomer to the
modified surface with the activated initiator present grows a thin
film at a rate of polymerization where the thin film has an average
chain molecular weight proportional to the rate of
polymerization.
15. The method of claim 11 where reacting the monomer to the
modified surface with the activated initiator present grows a
polymer film on the substrate with a film thickness linearly
proportional to reaction time.
16. A method for forming organic thin films using surface initiated
vapor deposition-polymerization of monomers comprising: providing a
substrate having a surface; providing an initiator; predepositing
the initiator on the surface of the substrate; disposing the
modified substrate in a vacuum containing the monomer; activating
the initiator; vaporizing the monomer; and reacting the monomer to
the modified surface with the activated initiator present, where
reacting the monomer to the modified surface with the activated
initiator present forms a polymer film by means of
nitroxide-mediated free radical polymerization with dormant
alkoxyamine groups at the chain ends of the formed polymers
comprising the polymer film, which is capable of re-initiating
polymerization to create a second block of polymer when the
reaction conditions are resumed.
17. The method of claim 16 further comprising sequentially
polymerizing additional blocks of copolymers on the polymer
film.
18. The method of claim 17 where sequentially polymerizing
additional blocks of copolymers on the polymer film produces a
hydrophilic-hydrophobic-hydrophilic alternating polymer thin film.
Description
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to the field of nitroxide mediated free
radical polymerization of vaporized vinyl monomers, including
acrylic acid (AAc), styrene (St), N-2-(hydroxypropyl)methacrylamide
(HPMA) and N-isopropyl acrylamide (NIPAAm), on silicon wafers.
2. Description of the Prior Art
Fabrication of polymer ultrathin films with controllable surface
properties is critical in many important industrial applications
and academic research. As device sizes continue to shrink, the use
of surface-initiated polymerization, where polymer films were
directly polymerized from surface premodified initiator layers, has
become increasingly important over conventional coating methods.
The polymer films fabricated via the surface-initiated
polymerization are end-grafted monolayers with superior
chemical/mechanical stability and controllable grafting thickness
and density. Typically, polymerization schemes were directly
adapted from the already existed chemical synthetic schemes for
their counterpart polymeric materials. However, in a
surface-initiated polymerization scheme, the initiators are crowded
on a two-dimensional surface and the polymerization only take place
at interfaces. This inherent nature has often raised difficulty in
synthesizing high molecular weight polymer products in that the
effective monomer to initiator ratio near surface is lower, while
the premature termination caused by impurities or side reactions in
solution are more dominant than the typical polymerization where
both monomers and initiators are evenly dispersed in the media.
Previously, empirical methods such as adding excess initiator
molecules in solution phase were proposed to improve the yield.
The prior art has described the synthesis of polymer brushes by
living free radical polymerization using another similar
alkoxylamine initiator, but it was performed in liquid phase. The
reaction performed in liquid is quite different from that in gas
phase. There have been some limited work on the synthesis of
polymer (polypeptide) brushes by living polymerization using quite
different initiators. Besides they used condensation polymerization
techniques instead of addition polymerization in our system.
As with preparation of self-assembled monolayers (SAMs), polymer
brushes are typically formed by first depositing initiating groups
on a substrate surface that covalently bind thereto. Then,
macromolecular chains are grown from the initiating groups using
monomers that are typically similar to those traditionally used in
microlithography, e.g., t-butyl acrylate. The covalent bonding of
the macromolecular chains to the substrate surface opens up a
number of possibilities that are not available with traditional
spin-cast films. These advantages permit the use of these films in
technological applications that include specialty photoresists,
sensors and microfluidic networks.
A number of different approaches to synthesis of patterned polymer
brushes have been described. For example, some have reported the
patterning of surface bound initiators by either photoablation or
photoinitiation, followed by polymerization to give discrete areas
of polymer brushes, while others have detailed the growth of
patterned polymer films using layer by layer techniques. In
addition, a number of groups have also reported the elaboration of
microcontact printed thiol monolayers to provide patterned polymer
brushes.
In the past, studies have been done on graft polymerization for
surface modification, but the main difficulty was the poor
controlling of composition, architecture and function of the
polymer layer. The appearance of living polymerization provided the
chance of changing this situation. In the recent years, efforts
have been made by the combination of graft polymerization and
living polymerization, some have succeeded. Usually they were not
highly efficient in initiating graft polymerization and applicable
to various monomer systems. Most importantly, they were not good in
patterning the polymer layer due to the limitation of solvent in
most systems.
Ever since the nitroxide-mediated radical polymerization was
proposed by M. K. Georges et al in 1994, it has been widely
investigated in many polymeric systems. It is a very attractive
approach to synthesize not only living homopolymers but also
block-copolymers, as a result of its living characteristic. More
recently, this approach has also been used to synthesize polymer
from an immobilized TEMPO initiator layers at surfaces, thus
creating an end-grafted polymer thin film layer as shown by
Husseman, et al in 1999.
BRIEF SUMMARY OF THE INVENTION
In the illustrated embodiment of the invention vaporized acrylic
monomers are used, as opposed to the solvated monomers, as the
source for synthesizing end-grafted block copolymer ultrathin films
on solid substrates. Conceptually, polymerization at vapor provides
a means to reduce the consumption of solvents, to eliminate time to
purge off O.sub.2, to shorten the reaction time, and to more easily
pattern the surfaces.
The illustrated embodiment is directed to a fabrication method for
organic ultrathin films (1.about.100 nm) by utilizing vapor
deposition polymerization in vacuum. A variety of polymer brushes
grafted on silicon oxide surfaces are fabricated through the living
polymerization of vaporized vinyl monomers from the surface
initiator layer. In particular, a derivative of 2,2,6,6-tetramethyl
piperidinyloxy (TEMPO) based alkoxylamine containing
trimethoxysilyl is pre-immobilized on the silicon (100) wafer with
the TEMPO group at the free end, which is applied for initiating
the growth of the grafted polymer layers from the surface via
living free radical polymerization. This polymerization is
performed in vapor phase instead of the conventional solution
phase. To monitor the chemical structures, growth of films, and the
surface energy, Fourier transform infrared spectrum (FTIR),
ellipsometry and contact angle goniometry are employed. It is found
that a thickness up to sub-micron is attainable within less than 2
hours. A nearly linear relationship between the polymer film
thickness and the reaction time enables an easy and exact control
of the resulting polymer thickness. In addition, the polymers with
various chemical functional groups, including phenyl, carboxyl,
amide, and hydroxyl, are successfully fabricated followed by the
same protocol. The fabricated Poly N-isopropyl acrylamide (PNIPAAm)
film also exhibits the unique reversible thermo-sensitive feature
of a homopolymer in aqueous system. Corresponding to the decrease
of temperature across the lower critical solution temperature
(LCST) of PNIPAAm, the thickness of PNIPAAm layer extended more
than 50% due to the phase transition. Besides, the living character
of this polymerization process allows the fabrication of not only
di-block copolymers, but also tri-block copolymers such as
PAAc-b-PSt-b-PHPMA, demonstrating the feasibility of exact control
of surface polymer composition and morphology at nanoscale.
The invention relates generally to: 1. a fabrication method for
creating polymer thin films with controlled properties; 2. a
fabrication method utilizing free radical polymerization of
vaporized vinyl monomers in vacuum to synthesize wide variety of
polymer thin films; 3. a fabrication method combined with
photolithography for creating surface patterns; 4. a composition of
matter for polymeric materials including polystyrene (PS), polymers
with functional groups such as hydroxy, carboxy, and amide, and/or
block copolymers composed of more than two of the above mentioned
polymers; 5. a thin film with a thickness controlled from 1 nm up
to submicron sizes; and 6. a method for creating "smart surfaces",
where surface properties can be regulated through the environment
stimulants and become fully reversible/recyclable as a product.
The invention relates to the synthesis of graft functional polymers
and block copolymers on solid surface in vapor phase. Using the
specially synthesized alkoxylamine initiator covalently immobilized
on silicon wafer to initiate living free radical polymerization in
vapor phase, functional polymers or block copolymers layers are
fabricated. The purpose or the invention is to synthesize graft
polymers with well-defined composition, architecture and function
for surface modification. The obtained surface layer not only
contains the designed surface properties such as hydrophobicity or
hydrophilicity and functional groups, but also has precise
structure or even pattern. It is applicable to most vinyl monomers.
It is a kind of nitroxide-mediated living free radical
polymerization, and based on a vapor deposition technique, it is
performed in vapor phase instead of conventional liquid phase.
The method of the invention is applicable to most of the vinyl
monomers, which offers more opportunity in fabricating polymer
layers with various functions. The stimuli-responsive polymer
layers can be obtained easily. The thickness of the fabricated
polymer is well controlled from a few nanometers up to submicron
thicknesses. Block copolymers are easily produced. Finally, the
solvent free process due to the vapor phase polymerization greatly
favors the surface patterning since it avoids the adverse effect on
photomasks.
One of the promising fields for the present invention is in the
field of biochips. The capability of precisely controlling
composition and architecture of surface polymer layers together
with the fine patterning techniques can produce chips with
well-patterned biomolecules for diagnosis and treatment. It is
advantageous also for patterning on silicon and metals with
nanometer thickness which opens a large market in microelectronics.
Besides, the invention can be used in many other areas such as
microfluidics, separation, optics, and the like.
One advantage of the invention compared to the prior art methods is
that the invention produces a product which is a much more densely
packed, self-regulating (sensing), and fully recyclable. The
resulting polymer thin films can be widely used for biochips,
coatings on biomedical devices for improving biocompatibility, for
coating a surface for antifouling, and anti-corrosion.
While the apparatus and method has or will be described for the
sake of grammatical fluidity with functional explanations, it is to
be expressly understood that the claims, unless expressly
formulated under 35 USC 112, are not to be construed as necessarily
limited in any way by the construction of "means" or "steps"
limitations, but are to be accorded the full scope of the meaning
and equivalents of the definition provided by the claims under the
judicial doctrine of equivalents, and in the case where the claims
are expressly formulated under 35 USC 112 are to be accorded full
statutory equivalents under 35 USC 112. The invention can be better
visualized by turning now to the following drawings wherein like
elements are referenced by like numerals.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a graph showing the FTIR spectra for silicon wafers
grafted with line (A) corresponding to PAAc (30 nm, reaction time:
18 min), line (B) corresponding to PSt (150 nm, 2 h), line (C)
corresponding to PHPMA (12 nm, 2 h), and line (D) corresponding to
PNIPAAm (29 nm, 5 h).
FIG. 2 is a graph showing the dependence of the thickness of PAAc
grafted on silicon wafer on polymerization time
FIG. 3 is a graph of the FTIR spectra of the grafted thin film
samples after sequential polymerizations. Line (A) shows the
spectrum after first polymerization from the TEMPO initiator: PAAc
homopolymer (30 nm, 18 min), and Line (B) shows the spectrum after
second polymerization from the PAAc film: Di-block PAA (30 nm, 18
min)-b-PSt (25 nm, 1 h), and line (C) shows the spectrum after
third polymerization from the diblock film: Tri-block PAA(30 nm, 18
min )-b-PSt (25 nm, 1 h)-b-PHPMA (10 nm, 3 h).
FIG. 4 is a graph which shows the change of the thickness and
refractive index of a grafted PNIPAAm film with time in water when
the temperature decreases from 50.degree. C. to 22.degree. C. The
measured thickness of the PNIPAAm film is 65 nm in air.
FIG. 5 is a table summarizing the vaporized acrylic monomers used
in the illustrated embodiment.
FIG. 6 is a table summarizing the fabrication of end-grafted
homopolymer thin films using nitroxide mediated radical
polymerization in the illustrated embodiment.
FIG. 7 is a table summarizing the fabrication of block copolymer
thin films using nitroxide mediated radical polymerization in the
illustrated embodiment.
FIG. 8 is a simplified diagram showing a device used for vapor
polymerization.
FIG. 9 is a graph showing contact angle as a function of
polymerization time demonstrating the heating effect of the TEMPO
modified silicon substrate for polymerization of PAA.
FIG. 10 illustrates the ellipsometric thickness of the film as a
function of polymerization time of PAA.
The invention and its various embodiments can now be better
understood by turning to the following detailed description of the
preferred embodiments which are presented as illustrated examples
of the invention defined in the claims. It is expressly understood
that the invention as defined by the claims may be broader than the
illustrated embodiments described below.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In the illustrated embodiment a vapor phase reaction scheme is used
to synthesize grafted polymer thin films. Inspired by the success
of gas phase reactions, such as chemical vapor deposition (CVD)
evaporation, and sputtering, in semiconductor and metallic thin
film fabrication, a vapor phase scheme, if chemically feasible, has
many advantages over the conventional solution phase reactions in
synthesizing high molecular weight thin film materials.
First, the vacuum environment can greatly eliminate impurities and
solvent molecules, thus prolonging the mean free path of vaporized
monomers reaching down the initiator-modified surfaces.
Second, a more efficient surface reaction can be facilitated in
that vaporized monomers possess higher thermal energy and can be
directional if desired.
Third, the reaction parameters such as the evaporating temperature
of monomers, the substrate temperature, degree of vacuum,
concentration of monomers, type of monomers, etc, can be adjusted
independently and quickly in a vapor phase reaction. Potentially,
the vapor phase reaction could be a more versatile method in
fabricating polymer films with patterns and multiple compositions
in three dimensions. This argument has been experimentally
confirmed in the case of ring-opening polymerization of N-carboxy
anhydrides (NCAs) of amino acids from the surface initiators. When
the parameters were optimized, the thickness of the grafted
polypeptide film synthesized via the so-called "surface initiated
vapor deposition-polymerization" (SI-VDP) is 10-fold higher than
those were produced in the solution phase polymerization.
To demonstrate the feasibility of SI-VDP of monomers which were
typically in solvated or liquid forms previously, we have chosen
nitroxide-mediated free radical polymerization of vinyl monomers as
the model system as diagrammatically summarized in FIG. 5. This
reaction scheme has been demonstrated as a living (or "controlled")
polymerization in solution phase for some industrially important
monomers such as styrene, and its surface-initiated protocol has
been successfully developed previously. Hence, the selection allows
us to investigate both synthetic capability as well as the
"controlled" feature of this important polymer category in an
SI-VDP setup. To do so, various vaporized vinyl monomers, including
styrene, acrylate and acrylamide, were used to synthesize both
homo- and block co-polymer brushes as described below.
The initiator, 1-(4-oxa-2'-phenyl-12'-trimethoxysilyl
dodecyloxy)-2,2,6,6-tetramethyl-piperidine (I) (TEMPO), was
synthesized and pre-deposited on a silicon (100) native oxide
surface via the silanol condensation reaction of trimethoxysilane
head groups. The modified silicon substrate was placed in a vacuum
chamber containing a small amount of monomers. At least 8 mm
displacement between the substrate and monomer source was placed to
ensure no direct contact. The chamber was evacuated under 10.sup.-3
Torr and sealed, and the temperature was elevated at 125.degree. C.
to activate TEMPO initiators, and to vaporize monomers. After the
reactions, the samples were cleaned thoroughly to remove
loosely-bound physisorbed materials followed by conventional
cleaning procedures.
Poly(acrylic acid) (PAAc) brushes were first fabricated on silicon
wafers by the SI-VDP of vaporized acrylic acid (AAc) monomers. The
successful fabrication of the film was confirmed by its
transmission Fourier transform infrared spectrum (t-FTIR), which
has the identical characteristic peaks such as the strong
absorbance at 1720 cm.sup.-1 attributed to carboxylic side chains
to the standard PAAc material. Line (A) in FIG. 1 shows the t-FTIR
spectrum of one particular grafted PAAc sample synthesized by the
SI-VDP for 18 min. The corresponding film thickness of the grafted
sample measured by Ellipsometer is 30 nm.
Other monomers, including styrene (St),
N-(2-hydroxypropyl)methacrylamide (HPMA) and, N-isopropyl
acrylamide (NIPAAm), were also applied in the SI-VDP scheme. Lines
(B), (C), and (D) in FIG. 1 are the spectra of the corresponding
polymer films respectively. The characteristic peaks from each
spectrum match the corresponding polymer standard, thus confirming
the feasibility of applying these vaporized monomers in a
TEMPO-initiated radical polymerization. Simultaneously, X-ray
photoelectron spectroscopy (XPS) was employed to examine the
surface composition of grafted PAAc, PSt, and PHPMA films, and the
surface elemental analysis based on the individual XPS scans is
summarized in Table 1. The experimental compositional ratios are in
good agreement with the theoretical stoichiometric ratios of each
polymer species, further confirming the results from FTIR.
TABLE 1 XPS data of polymers grafted on silicon wafer Surface
composition Reac- Ellipsometric N.sub.C(1s) :N.sub.N(1s)
:N.sub.O(1s) : N.sub.Si(2p) tion thickness XPS Theoretical value of
pure Sample time (nm) experimental polymer sample PAAc 0.5 h 61
61:0:39:0 60:0:40:0 PSt 1 h 84 74:0:18:8 100:0:0:0 PHPMA 5 h 26
67:10:23:0 70:10:20:0 * The presence of O(1s) is attributed by the
silicon oxide substrate (SiO.sub.x), indicating the presence of
defects in this particular sample.
We also performed the surface polymerization of abovementioned
monomers in solution phase in order to compare the results with
those from the SI-VDP schemes. By adjusting the reaction parameters
appropriately, we were able to fabricate grafted PMc, PSt and PHPMA
thin films in both vapor and solution phase with comparable
results. However, to date we have not been able to produce grafted
PNIPAAm films in any circumstances. For example, we have tried the
polymerization in different solutions including water, alcohol,
toluene, and dioxane, but none of them generates surface-bound
PNIPAAm brushes. This suggests that the SI-VDP technique does offer
unique advantages for the polymer systems that are difficult or
impossible to obtain in conventional solution phase.
With the successful fabrication of grafted PNIPAAm via the SI-VDP,
for the first time, we are able to study the temperature response
of this thermal-sensitive polymer in an end-grafting state. PNIPAAm
is known to reversibly expand when temperature is below its lower
critical solution temperature (LCST) in the aqueous solution.
Therefore, a film of grafted PNIPAAm is anticipated to change its
dielectric properties (such as film thickness and refractive index)
with temperature. Indeed, using ellipsometry to measure the
thickness of the PNIPAAm film in situ, we found that the solvated
PNIPAAm film with an original thickness of 120 nm (>32.degree.
C.) can expand over 200 nm (<32.degree. C.) below its LCST point
as illustrated in FIG. 2.
The kinetic plot of ellipsometric film thickness of the resulting
grafted PAAc film versus polymerization time, as shown in FIG. 2,
demonstrates that the SI-VDP via nitroxide-mediated polymerization
scheme is effective in synthesizing PAAc films. Within a 2 h
reaction, a film of nearly 200 nm thickness was obtained. While the
monomer concentration in solution polymerization decreases as the
reaction progresses, in a SI-VDP setup, the vaporized monomer
concentration remains constant throughout the reaction. According
to Rault's law, as far as there is condensed monomer in excess, the
monomer is saturated at vapor phase in equilibrium state.
Accordingly, the average chain molecular weight of the film is
proportional to the rate of polymerization.
As shown in FIG. 2, within 2 h, the polymer thickness and reaction
time remains linearly proportional, confirming our hypothetical
model. The linear relationship allows one to control the resulting
thicknesses by controlling the reaction time. FIG. 10 illustrates
the ellipsometric thickness of the film as a function of
polymerization time of PAA.
Although we have not fully optimized the reaction conditions for
each polymer system, the current results show that the grafting
efficiency is highly dependent upon the side chain groups. For
example, the grafted PAAc or PSt films required less time than the
grafted PHPMA or PNIPAAm films to reach the same thickness level,
i.e. 150 nm thickness of PAAc or PSt film was generated in 2 h.
One unique feature of nitroxide-mediated free radical
polymerization is the presence of dormant alkoxyamine groups at the
chain ends of the formed polymers (mainly styrene-based polymers),
which is capable to re-initiate polymerization to create a second
block of polymer when the reaction conditions are resumed.
Because such a "living" characteristic is important toward
controlling surface composition and morphology at nanoscale, it
would be of great interest if the SI-VDP protocol also remains
"renewable". In our case, we conducted SI-VDP for multiple cycles
to demonstrate its renewability. By two sequential polymerization,
the amphiphilic monolayer composed of grafted diblock copolymers of
PAAc (the 1.sup.st layer)-b-PSt (the 2.sup.nd layer), or PSt (the
1.sup.st layer)-b-PAAc (the 2.sup.nd layer) were fabricated. More
strikingly, by three sequential polymerization, the grafted
triblock copolymer of PAAc (30 nm, the 1.sup.st layer), PSt (25 nm,
the 2.sup.nd layer) and PHPMA (10 nm, the 3.sup.rd layer) was
demonstrated. The t-FTIR spectra in FIG. 3 show the sequential
formation of the triblock copolymer. The water contact angles of
the surfaces after each polymerization cycle, as indicated in Table
2, are the complementary evidence of successful grafting of each
layer: The grafting of PAAc or PHPMA as the outmost layers led to a
hydrophilic surface, while PSt led to a hydrophobic one. The
creation of a hydrophilic-hydrophobic-hydrophilic alternating
polymer thin film clearly confirms the renewal capability of
TEMPO-initiated polymerization at vapor phase.
TABLE 2 Advanced water contact angle after sequential surface
reactions Clean Si TEMPO Treatment wafer Initiator PAAc.sup.a
Diblock.sup.b Triblock.sup.c Contact 10 .+-. 2 60 .+-. 2 40 .+-.
2.5 90 .+-. 2.5 49 .+-. 2.5 angle (.degree.) .sup.a.about.c Vapor
polymerization for the synthesis of .sup.a PAAc (30 nm). .sup.b
PAAc (30 nm)-b-PSt (25 nm). .sup.c PAAc (30 nm)-b-PSt (25
nm)-b-PHPMA (10 nm).
In summary, we have successfully demonstrated the applicability of
nitroxide mediated polymerization of vaporized vinyl monomers.
Through this SI-VDP process, grafted PAAc thin films with
thicknesses from few nanometers to submicrons were fabricated
within hours. Interestingly, its linear relationship between
thickness and reaction time allows one to further predict and
control the resulting film thickness.
Furthermore, other thin films of homopolymers (PSt, PHPMA, and
PNIPAAm) as diagrammatically depicted in FIG. 6 and block
copolymers, including the triblock copolymer of PAAc-b-PSt-b-PHPMA,
as diagrammatically depicted in FIG. 7 were also obtained
successfully.
Finally, the combination of solvent-free process and
surface-initiated polymerization does provide not only an
environmentally cleaner and more efficient technique for
fabricating polymeric thin films than the existing solution
polymerization, but also a more flexible protocol for surface
patterning. In analogy to the vapor phase process in semiconductor
manufacturing, the conventional photolithographic techniques are
completely applicable for this fabrication process for organic thin
film synthesis, as it avoids the adverse effect on photomasking
that usually arises from the interference of solvents.
AN EXAMPLE
An experimental example of one embodiment will make the invention
clear. A silicon wafer was cleaned with H.sub.2 O.sub.2 /H.sub.2
SO.sub.4 (3/7, v/v) and immersed in solution of initiator in
anhydrous toluene for 2 h at room temperature. The TEMPO initiator
was tethered on the silicon oxide surface 10 through the silanol
condensation reaction and confirmed by ellipsometry and contact
angle measurements. After extensive washing and drying, the
initiator immobilized silicon wafer 12 mounted on a metal plate 22
was placed into a reaction chamber 14 containing small amount of
monomers 16 as shown diagrammatically in FIG. 8. The temperature of
plate 22 and hence wafer 12 was monitored by a thermocouple 18 and
a heater coil 20 was thermally coupled to plate 22 to precisely
control the temperature of wafer 12. Oxygen in that reaction
chamber 14 was removed completely by repeating at least three
cycles of evacuating and then purging with nitrogen. Finally the
reaction chamber 14 was evacuated to about 1.times.10.sup.-3 Torr,
sealed and transferred to an oven or oil bath at 125.degree. C. for
a designed period. After the reaction, the silicon wafer 12 was
cleaned thoroughly with appropriate solvents to remove
non-covalently bound species. FIG. 9 illustrates the importance of
temperature control of wafer 12 where full coverage of a PAA film
can be synthesized in just 5 minutes if wafer 12 is maintained in
the range of 90 to 100.degree. C. as opposed to more than 40
minutes if there is no wafer heating.
The surface-grafted PNIPAAm (65 nm in air) was put into a liquid
cell full of water at high temperature (higher than its LCST).
During the decrease of temperature with time, the change of PNIPAAm
thickness and refractive index in water was measured by
ellipsometry as illustrated in FIG. 4.
Many alterations and modifications may be made by those having
ordinary skill in the art without departing from the spirit and
scope of the invention. Therefore, it must be understood that the
illustrated embodiment has been set forth only for the purposes of
example and that it should not be taken as limiting the invention
as defined by the following claims. For example, notwithstanding
the fact that the elements of a claim are set forth below in a
certain combination, it must be expressly understood that the
invention includes other combinations of fewer, more or different
elements, which are disclosed in above even when not initially
claimed in such combinations.
The words used in this specification to describe the invention and
its various embodiments are to be understood not only in the sense
of their commonly defined meanings, but to include by special
definition in this specification structure, material or acts beyond
the scope of the commonly defined meanings. Thus if an element can
be understood in the context of this specification as including
more than one meaning, then its use in a claim must be understood
as being generic to all possible meanings supported by the
specification and by the word itself.
The definitions of the words or elements of the following claims
are, therefore, defined in this specification to include not only
the combination of elements which are literally set forth, but all
equivalent structure, material or acts for performing substantially
the same function in substantially the same way to obtain
substantially the same result. In this sense it is therefore
contemplated that an equivalent substitution of two or more
elements may be made for any one of the elements in the claims
below or that a single element may be substituted for two or more
elements in a claim. Although elements may be described above as
acting in certain combinations and even initially claimed as such,
it is to be expressly understood that one or more elements from a
claimed combination can in some cases be excised from the
combination and that the claimed combination may be directed to a
subcombination or variation of a subcombination.
Insubstantial changes from the claimed subject matter as viewed by
a person with ordinary skill in the art, now known or later
devised, are expressly contemplated as being equivalently within
the scope of the claims. Therefore, obvious substitutions now or
later known to one with ordinary skill in the art are defined to be
within the scope of the defined elements.
The claims are thus to be understood to include what is
specifically illustrated and described above, what is
conceptionally equivalent, what can be obviously substituted and
also what essentially incorporates the essential idea of the
invention.
* * * * *